In the manufacture of integrated circuits, deposition of films or materials with non-conformal properties over retrograde topography often yields subsequent structures which also have retrograde or re-entrant profiles. Subsequent depositions of non-conformal and other films over re-entrant profiles frequently result in structures having an "overhang" which obscures the underlying topography.
Retrograde profiles may result from the deposition of: non-conformal films over ideally vertical profiles, and conformal films over profiles which are already retrograde.
Some manufacturing applications involve depositing a material over features which may have retrograde or re-entrant profiles, and then patterning and etching the material. One example is the anisotropic etching of polysilicon over oxide.
Anisotropic etches (i.e., etches exhibiting no significant undercut) are used to prevent significant critical dimension loss. "Critical dimension" refers to the distance between features.
Anisotropic etches remove material in a direction substantially perpendicular to the plane defined by the substrate. The material which is etched must therefore be in the "line of sight" of the plasma, as viewed from a point directly "above" the feature. Consequently, any overhanging topography shelters the material which is anisotropically etched. If conductive materials, such as polysilicon, form the layers, then conductive shorts may result between adjacent features as a consequence of the residual sheltered material. The residual material is referred to as a "stringer," as seen in FIG. 1.
"Stringers" can be defined as residual material resulting from an etch process. The "stringers" are a problem when they are comprised of a conductive material which causes short circuiting between adjacent structures.
Current stacked capacitor dynamic random access memories (DRAMS) often comprise components which have high, vertical (retrograde) topologies. Such topologies are susceptible to "stringers" following etch steps. Since DRAMs are comprised of polysilicon, which is a conductive material, any "stringers" may result in non-functional parts and consequent yield loss.
Furthermore, the etching of sub-half micron structures from polysilicon requires a process having a high degree of selectivity and anisotropy, without damaging the structure being formed.